Here we analyze the role of monocyte/macrophage derived NF-κB in autoimmune demyelination. We studied EAE in mice which are devoid of IκBα in myeloid cells (lysMCreIκBαfl/fl mice). In these mice, loss of IκB in monocytes and macrophages leads to constitutive expression of NF-κB. In turn, this results in an increased expression of NF-κB regulated monocyte/macrophage cytokines and subsequently enhanced macrophage infiltration and iNOS expression in the spinal cord of EAE mice. These mechanims govern demyelination, enhanced axonal damage and finally a more severe course of MOG-EAE. Thus macrophage derived, NF-κB dependent cytokines may play a pivotal role in the pathogenesis of EAE and determine the outcome of autoimmune inflammation in the CNS without interfering with Th1 and Th17 T-cell responses. Our findings suggest that NF-κB in myeloid cells is a master regulator for regulation of inflammation and tissue damage in autoimmune inflammation of the CNS.
Previous studies already investigated the role of NF-κB in the CNS , in autoimmune diseases  and also during EAE. The analysis of c-Rel or NF-κB1-deficient mice as well as IKK-2-deficient mice revealed that NF-κB activation in T-cells significantly contributes to the initiation of autoimmune neuroinflammation [17, 18]. Myelin-specific T-cells in NF-κB1 (p50)-deficient mice are deficient in differentiating into either Th1 or Th2 cells . This concept was further refined by Dasgupta and co-workers who showed positive effects of NF-κB on the differentiation of myelin-specific Th1 cells, but a negative influence on the differentiation into Th2 cells . In line with these observations, severely impaired T-cell responses were found in immune cell cultures derived from mice with a T-cell specific deficiency in IKK2, a pivotal kinase for NF-κB activation (IKK2Δ T-cell mice) .
Moreover, NF-κB may also play a role in the CNS cells during autoimmune demyelination. While studies by Wooten or Mattson and co-wokers in cell culture argue for a protective role of NF-κB in neuronal cells [21, 22], our work with conditional overexpression of NF-κB in myeloid cells reveal monocytes/macrophages as a further cell type with crucial importance of NF-κB in neuroinflammation.
Many studies have demonstrated a critical role for macrophages/microglia as well as an up-regulation of MHC class II in these cells in EAE and MS lesions [23, 24]. Depletion experiments with liposomal dichloromethylene diphosphonate (Cl2MDP) revealed a crucial role of bone marrow derived macrophages for tissue damage in EAE [25, 26]. Interestingly, histological analyses in these studies could not detect differences in number and localization of CNS infiltrating T-cells between Cl2MDP treated rats and controls . These data argue for a myeloid cell independent T-cell activation in EAE. Well in line with this concept, we did not observe any change in T-cell cytokine expression in our model. Indeed, not monocytes/macrophages, but rather dendritic cells as professional antigen presenting cells do play the major role for T-cell activation in EAE . Furthermore, lysMCre mediated IκBα deletion in our model does not involve dendritic cells and does not lead to effects on T-cell cytokines. Thus at first glance, NF-κB in macrophages/monocytes may be more important for effector functions than for T-cell activation. Yet, changes in phagocyte cytokine patterns may eventually result in an enhanced T-cell activation over time which is not reflected in our analysis at day 10 after immunization. Further studies are warranted to dissect indirect effects of myeloid cell derived NF-κB on T-cell function in the setting of chronic inflammation.
Our data reveal a crucial role of myeloid cell derived NF-κB for demyelination. In our model, demyelination or oligodendrocyte apoptosis may be mediated by a direct phagocytic attack of macrophages or also by macrophage derived toxic cytokines. Here, especially TNF-α as a typical NF-κB regulated cytokine in macrophages may play a role . Previous studies already indicated a major role of TNF-α for macrophage recruitment from the periphery  and also for oligodendrocyte apoptosis as well as toxic demyelination, especially in the MOG-EAE models of the C57BL/6 mouse [30, 31]. Finally, myelin loss might also be influenced by free radicals. As shown, overexpression of NF-κB may lead to an increased expresion of the NF-κB target gene inducible NO-synthase (iNOS) and thus an increased NO production which was previously shown to exert a detrimental role on oligodendrocytes and in EAE [32, 33].
Moreover, reactive oxygen species (ROS) intermediates may induce cellular damage and trigger demyelination as well as a recently shown reversible axonal damage called focal axonal degeneration (FAD) . In this process, ROS mediated NF-κB activation  and in turn NF-κB induced further ROS production in macrophages may play an important role. This notion is underscored by the fact that macrophages were found to play an important role in the process of ROS mediated FAD .
In our model, further pathways in NF-κB activation may be involved. Among others, these factors may include toll-like receptors (TLR). In EAE, especially TLR9 on microglia may play an important role for interaction with invading immune cells and for the initiation of a cascade resulting in NF-κB activation and finally an enhanced innate immune responses .
Theoretically, NF-κB overexpression in macrophages may also exert protective effects in EAE. Indeed, overexpression of the NF-κB target gene "triggering receptor expressed on myeloid cells 2" (TREM2) in myeloid cells lead to an enhanced expression of anti-inflammatory cytokines in the spinal cord of EAE mice as well as a reduced amount of demyelination and axonal damage . In our setting, overexpression of NF-κB in macrophages predominantly induces destructive effects thus arguing for the prevailing importance of NF-κB mediated pathways for detrimental phagocyte functions in EAE.